Electrotaxis as used herein means the involuntary movement of fish in an electric field toward the positive electrode. This condition is often utilized in research and resource management to inventory, inspect, and mark fish. Assuming the electric field has been properly regulated for the given body of water, in order to initiate electrotaxis, sufficient voltage must be available in the water adjacent the positive electrode to momentarily stun the fish and enable researchers to gather them with nets or the like. At the same time, however, the voltage must not be so great as to permanently harm the fish.
Different bodies of water may have markedly different abilities to conduct electricity therethrough, and indeed the same body of water may exhibit significant differences in conductivity in different locals. For instance, a relatively clean portion of a fresh-water lake, such as in the center of the lake least subject to contamination by runoff may be quite clean and offer significant resistance to an electric current induced therein. However, in the portion of the same freshwater lake subject to contamination by a mineral spring, creek or river, quite high conductivities may be experienced. Likewise, temperature variation in a body of water can dramatically alter the conductivity. For instance, the resistance of cold tap water (about 50.degree. F.) is on the order of 3,000 ohms, while the resistance of the same water warmed to about 68.degree. F. decreases to about 1,500 ohms.
Fisheries' biologists have a need to ascertain fish population data through methods which permit recovery of a significant percentage of fish, without permanently damaging the fish. "Electrofishing" has become the method of choice for such fish collecting as it is relatively easy and effective. However, a significant problem exists in the proper application of the electric field to the body of water so as to apply the optimum current to collect the maximum number of fish for analysis without damaging or killing any of such fish. Heretofore, operators of the electrofishing equipment have had no means to test a given body of water within which electrofishing was to take place. They simply applied a voltage field to the body of water and presumed it would be strong enough to induce electrotaxis but not so strong as to damage fish.
Conductivity meters have long been available for laboratory use. In general, such meters do not provide an actual conductivity reading, but rather a standard conductivity reading which is corrected to 25.degree. C. With reference to a standardized chart, an indication of the relative conductivity of the particular water tested is obtained. Experience indicated that the initial voltage applied in the field was often incorrect and either no electrotaxis occurred, or fish were killed or damaged due to the application of excessive voltage to the body of water.
The conductivity of potable waters in the United States generally ranges from about 20 to about 1,500 micromhos/cm. Exceedingly clean water exhibits very low conductivity, and higher voltages or an increase in the size of electrodes utilized, is, therefore, required in order to generate electrotaxis in fish. Maintaining a large cathode (boat hull) to anode ratio (10:1 minimum) permits the cathode to distribute the field over a larger area, and by electrotaxis, cause the fish to approach the anode. In waters of very low conductivity (less than 200 micromhos/cm) very high voltages must be used. If the biologists utilizing such voltages are moving through a body of water, and unknowingly enter a higher conductivity area, the high voltages used may kill many fish.
Therefore, it is an object of this invention to provide a portable hand-held apparatus which may be utilized to not only determine the conductivity of a localized area within a body of water, but which may also measure the field strength of the voltage applied to such water to ensure that a shocking voltage is within an optimum range. Such apparatus is provided with electronics enabling it to readily switch from measuring conductivity to measuring field strength, thereby providing operators of the voltage source with immediate information enabling more precise operation.
It is a further object of the present invention to provide a method for accurately measuring the actual conductivity of a body of water so that an optimum voltage electric field may be applied to the water, with the electric field being measured by the same apparatus, and the conductivity or field strength displayed.
These and other objects of the invention will become readily apparent with reference to the following description and claims.